using namespace std;
#include<cstdio>
#include<algorithm>
#define DIM 8192
char vec[DIM];
int pz;
typedef int ll;
const int MAX_N = 2048;
ll N, C, SOL;
struct client { ll t, p; };
struct cmp
{
	bool operator()(const client &a, const client &b)const
	{
		return a.t < b.t;
	}
};
client A[MAX_N];
inline ll max(ll a, ll b) { return (a>b)?a:b; }
void cit(int &x)   
 {   
  x=0;   
  while(vec[pz]<'0' || vec[pz]>'9')   
  
    if(++pz==DIM) fread(vec,1,DIM,stdin),pz=0;   
  
    while(vec[pz]>='0' && vec[pz]<='9')   
    {   
        x=x*10+vec[pz]-'0';   
        if(++pz==DIM)fread(vec, 1, DIM, stdin),pz=0;   
    }   
 } 
int main()
{
	freopen("carnati.in","r",stdin); freopen("carnati.out","w",stdout);
	cit(N), cit(C);
	ll i, j, cur, ant, P, V;
	for(i = 1; i <= N; ++i) cit(A[i].t),cit(A[i].p);
	sort(A+1,A+N+1,cmp());
	for(i = 1; i <= N; ++i)
	{
		P = A[i].p;
		if(A[1].p >= P) ant = P - C;
		else ant = 0;
		SOL = max(SOL, ant);
		for(j = 2; j <= N; ++j)
		{
			V = 0;
			if( A[j].p >= P) V = P;
			cur = max( ant + V - C * ( A[j].t - A[j-1].t ), V - C );
			SOL = max( SOL, cur );
			ant = cur;
		}
	}
	printf("%d\n",SOL);
	return 0;
}